1. Field of the Invention
The present invention relates to a surface modified material, a sulfonimide compound for surface modification, and a fuel cell, and more specifically, to a surface modified material prepared by bonding a sulfonimide compound onto a surface of a base material, a sulfonimide compound for surface modification for use in the production of such a surface modified material, and a fuel cell using such a surface modified material for a catalyst layer.
2. Description of the Related Art
Chemically bonding various kinds of organic compounds onto a surface of a base material allows imparting functions of the organic compounds and/or new functions to the base material. Moreover, in such a material prepared by chemically bonding an organic compound onto a surface of a base material (surface modified material), the organic compounds is unlikely to fall out, and therefore deterioration of imparted functions is characteristically low. A number of such surface modified materials and methods for producing the same have been heretofore suggested.
For example, Patent Document 1 discloses a carbon product obtained by adding a graphite powder and 4-chlorobenzene diazonium hexafluorophosphate to water, stirring the mixture for 20 minutes, filtering and drying the product.
The document describes that it is possible to obtain a graphite powder containing a chlorophenyl group without providing a current sufficient for reducing a diazonium salt from outside by such a method.
Patent Document 2 discloses a platinum-supported carbon prepared by having platinum particles supported on a surface modified carbon material with N,N-dimethyl amino phenyl group introduced onto its surface.
The document describes that modifying the surface of the carbon material with an organic group containing nitrogen atoms allows platinum fine particles to be highly dispersed on the surface of the carbon material and strongly supported onto the same.
Patent Document 3 discloses that a platinum-supported carbon prepared by having platinum particles supported on a surface modified carbon material with a nitrophenyl group, an anthraquinonyl group, or N-methyl phthalimide introduced onto its surface.
The document describes that modifying the surface of a carbon material with an organic group having a reducing potential on a high potential side higher than −2.5 V relative to SCE allows platinum fine particles to be highly dispersed on the surface of the carbon material and strongly supported onto the same.
Non-patent Document 1 discloses a material having benzene sulfonimide (—C6H4—SO2N−SO2CF3) electrochemically supported on the surface of carbon.
Patent Document 4 discloses a carbon catalyst having poly-p-sodium styrenesulfonate introduced onto a surface of a carbon material by grafting by allowing reaction between the carbon material in which its surface is covered with a silane coupling agent and p-sodium styrenesulfonate.
The document describes that the oxygen reduction activity of the carbon catalyst is improved by introducing ion-exchanging functional groups on the surface of the carbon catalyst.
Non-patent Document 2 discloses introduction of benzyl alcohol onto the surface of carbon, and a carbon material prepared by graft polymerization using the surface as a starting point. This document describes that such a carbon material can be utilized as a catalyst layer for fuel cells.
In addition, Patent Document 5 discloses polybutyl methacrylate-modified carbon black obtained by:
(1) synthesizing polymerization initiator-modified carbon black by introducing a polymerization initiator 2-bromoisobutyric acid-p-(bromomethyl)benzyl on the surface of carbon black, and
(2) allowing polymerization by adding butyl methacrylate and 4,4′-dinonyl-2,2′-bipyridyl to a dispersion of the polymerization initiator-modified carbon black.
This document describes that introducing a functional group (polymerization initiating group) having polymerization initiating ability into a methyl halide aromatic compound or its derivative allows introduction of a polymerization initiating group on the surface of various base materials having a nucleophilic functional group, and allows grafting of polymer chains at a high density onto the surface of the base material using the polymerization initiating group as a starting point.
A catalyst layer of a solid polymer fuel cell generally consists of a complex of carbon which supports an electrode catalyst, such as platinum, and catalyst layer ionomers. The catalyst layer ionomers generally use a fluorinated electrolytic polymer such as Nafion (registered trademark) therein. In order to improve the performance of the fuel cell, the higher the oxygen permeation and proton conductivity of the catalyst layer ionomers, the better.
However, known catalyst layer ionomers have low oxygen permeation, and therefore have the problem that oxygen reduction efficiency of Pt is low and high performance cannot be obtained. Moreover, there has been the problem that lowering the equivalent weight (EW) of the catalyst layer ionomers in order to improve proton conductivity makes the catalyst layer ionomers flow out due to water more easily.
In order to solve these problems, chemically bonding an electrolyte on a surface of a carbon carrier, that is, modifying the surface of the carbon carrier with the electrolyte is also possible. Methods for having an organic compound supported onto the surface of carbon generally involves, as described in Patent Documents 1 to 3 and Non-patent Document 1, using a diazonium salt. It is considered that the surface of the carbon carrier can be modified with the electrolyte by introducing a benzene ring onto the surface of carbon using a diazonium salt and further introducing an acid group into the benzene ring.
However, heretofore, it has been possible only to introduce a single acid group, or two acid groups at the most for a single benzene ring. Moreover, introducing many benzene rings (acid groups) into a graphite plane increases resistance, thereby lowering electric conductivity of graphite and performance. In addition, since the graphite plane wants to maintain its sp2 property, heat and/or potential scan cause benzene rings to leave the plane, and consequently cause acid groups to leave.
Meanwhile, grafting using no diazonium salt has also been reported (Patent Document 4, Non-patent Document 2). These techniques are thought to have low performance at a low humidity because the grafted material is a hydrocarbon material which has lower acidity than a fluorinated material does.